In plug and play systems using the Microsoft.RTM. Windows.RTM. operating system with plug and play capability, a problem arises in re-installing previously installed cards that use the Video Port Manager (VPM.TM.) and were subsequently removed.
For example, a user purchases a new video card and installs the card into the computer. During the installation process, the old video card, which also uses VPM.TM., is removed, however, the old drivers remain in the system. The system keeps track of the drivers in the system registry. The plug and play system notes the installation of the new card and requests the drivers for the new card. The user inserts the installation disk, which provides the instructions on configuring the drivers within the system so that a software client (or program) can use those drivers, and thus, use the new card. The installation process also inserts VPM.TM. information in accordance with the VPM.TM. specification into the Windows SYSTEM.INI file, so that software clients will know where to look for the VPM.TM. driver.
Thus, when only new and different cards are installed, VPM.TM.'s specified way of retrieving the driver information works with the plug and play system. However, if the user should remove the new video card, and re-install an old and previously installed video card, problems will arise. During the installation process of the old video card, the plug and play system will consult the system registry and realize that the system knows this card and already has the drivers for the card. Consequently, the system does not require loading of the drivers via the installation disk. But most importantly, the installation process does not replace the Windows SYSTEM.INI file entry for VPM.TM. because this file is not part of the plug and play subsystem. Thus, SYSTEM.INI file contains the instruction line for the new card and not the re-installed old card. Therefore, software clients will not run the correct drivers for the old card.
The result is that the client would obtain the driver for the new card and attempt to access the re-installed old card using the incorrect driver. If the driver is incompatible with the card, then, in the worst case, the system will crash, or in the best case, the card will operate erroneously. This will occur with other types of devices in addition to video cards.
Therefore, it is desired to have the client obtain the correct interface for the actual device which is in use even though the devices have been changed.